The present invention relates to a method of estimating in a receiver transmitted symbols from a transmitted signal in conjunction with the transmission of digital signals over a channel, wherein said symbol estimation is effected in accordance with a viterbi algorithm which has a number of states, said method comprising the following method steps:
receiving and filtering the transmitted signal to form a baseband signal;
sampling the baseband signal at at least two sampling time points for each symbol;
effecting correlation to determine the estimated impulse response of the channel with the aid of the sampled signal values and one or more known sequences;
determining a symbol sampling time point at one of the sampling time points;
selecting at least two of the sampling time points with each symbol, of which one is the symbol sampling time point, and selecting the sample signal values at these time points;
determining the delta-metric values in accordance with the viterbi algorithm for an indicated transmitted symbol, said determining process being effected for each of the selected sample signal values and for each state transition of the viterbi algorithm; and
generating at least preliminarily estimated symbols in accordance with the viterbi algorithm.
The invention also relates to an arrangement for carrying out the method.
One problem which often occurs when transmitting digital radio signals over a channel is that a transmitted signal is subjected to multipath propagation, resulting in time dispersion and noise. For instance, in mobile telephony, the channel transmission properties will change as a result of a mutual change in the positions of transmitter and receiver. These problems have been solved in time-shared, digital radio transmission systems, by giving the signal sequences, which are transmitted in a time slot, a synchronization sequence and a data sequence. The synchronization sequence is known to the receiver and with the aid of this sequence the receiver is able to make an appraisal of the channel transmission properties, a channel estimate. The receiver makes an appraisal of the symbols of the data sequence, which contains the information to be transmitted, with the aid of this channel estimate.
In certain cases, it is not sufficient to make a channel estimate only once with each time slot. In the case of long time slots, in the order of several milliseconds, transmitter and receiver are able to change their mutual positions quite considerably during the course of the time slot. This means that the channel transmission properties may change considerably over the duration of the time slot, such that the appraisal of the transmitted symbols made by the receiver will be deficient and the transmitted information therefore unclear. A radio receiver in which these disturbances are partially avoided is described in an article in IEEE Transactions on Information Theory, January 1973, pages 120-124, F. R. Magee, Jr. and J. G. Proakis: "Adaptive Maximum-Likelihood Sequence Estimation for Digital Signaling in the Presence of Intersymbol Interference". The article describes a channel equalizer comprising a viterbi analyzer which includes an adaptive filter as a channel estimation circuit. Received symbols are compared successively with hypothetical symbols and those hypothetical symbols which coincide closest with the received symbols are selected successively to form an estimated symbol sequence. The parameters of the adaptation filter are adjusted successively to the changed channel with the aid of the selected, decided symbols.
A description of the viterbi algorithm is found in an article by G. David Forney, Jr.: "The Viterbi Algorithm" in Proceedings of the IEEE, Vol. 61, No. 3, March 1973. The article describes in more detail the state of the viterbi algorithm and its state transitions and discloses how these state transitions are chosen so as to obtain the most probable symbol sequence.
Signal transmission between transmitter and receiver may be encumbered with certain deficiencies, despite carrying out sequence estimation and adaptive channel estimation in the aforesaid manner. One reason for these deficiencies is that the signal bandwidth exceeds the symbol rate of the system, which is commonly known as "excess bandwidth" as in the case, for instance, of the North American mobile telephone system ADC. Such systems are also known as "excess bandwidth systems". A solution to these excess bandwidth problems is described in an article by Yongbing Wan, et al, "A Fractionally-Spaced Maximum-Likelihood Sequence Estimation Receiver in a Multipath Fading Environment" Proceedings ICASSP pp. 689-692, published by IEEE 1992. According to this article, a received radio signal is sampled twice with each symbol and the channel estimation is effected through an adaptive filter which utilizes this double sampling rate. The symbol estimation is carried out in a viterbi analyzer which also utilizes the double sampling rate. The delta-metric values, i.e. deviations between the received and the hypothetical sequences, are calculated on both the sampling occasions for each symbol and the two delta-metric values are summed directly in order to determine a best state transition according to the viterbi algorithm. When adapting the filter with the aid of the estimated symbols, a fictive symbol is inserted at each alternate sampling time point. These fictive symbols are generated by interpolation between the estimated symbols in a second filter. The proposed solution has certain drawbacks. It is necessary to sample the received symbols at a time point which has been very well established and the adaptive channel estimation is of high complexity. The interpolation in the second filter results in delays which impair the symbol estimation. The signalling processing filters used, for instance a transmitter filter, or receiver filter must be known. The receiver filters in particular, which may include coils and capacitors, cause problems in this respect as a result of aging, manufacturing accuracy and temperature variations.
Another solution to the problems that occur at the aforesaid relatively low symbol frequency is given in a paper by R. A. Iltis: "A Bayesian Maximum-Likelihood Sequence Estimation Algorithm for A-Priori Unknown Channel and Symbol Timing", Department of Electrical and Computer Engineering, University of California, Santa Barbara, Aug. 21, 1990. This paper also states that sampling of a received signal shall be effected twice for each symbol. The symbol estimation is effected in accordance with a viterbi algorithm, which calculates two delta-metric values for each symbol, and these two values are co-weighted in the metric calculation. The channel estimation is effected in an adaptive filter with filter coefficients at a symbol time spacing, although the coefficients are adapted on each sampling occasion, thus twice with each symbol. The proposed solution further involves a relatively complicated metric calculation, and due to the symbol-spaced channel estimate and fails to solve the problem of symbol synchronization for complicated, rapidly varying channels. As with the aforedescribed solution proposed by Yongbing Wan, a receiver filter for instance must be known with good accuracy by the receiver.